Definition of Terms
For use herein, it is necessary to define several categories of silicone compounds. The categories are: silicone fluids, silicone greases, silicone plastics, silicone rubbers, curable silicone compositions and silicone antifoam agents.
An ideal fluid will flow when subjected to any magnitude of shear stress. However, silicone fluids sometimes resist some level of shear stress before flow is initiated. Hereinafter, a fluid will be identified as a material that flows when subjected to a shear stress of 10 Pa or less at 25.degree. C. A silicone fluid contains at least 90 percent diorganosiloxane.
Materials that will withstand significant shear stress without flowing will be categorized into two groups: greases and plastic solids. If a sufficient magnitude of shear stress is applied to these materials they will flow like a viscous liquid. The property of acting like a solid at low shear stress and like a liquid at some higher shear stress is called Bingham plasticity (see ENGINEERING RHEOLOGY, Clarendon Press, Oxford, 1985, pages 12 and 13 for a complete definition). The shear stress necessary to initiate flow will be referred to as the yield stress. Above the yield stress, an ideal Bingham plastic is expected to have a linear relationship between shearing stress and shear rate; here, a non-linear relationship between shearing stress and shear rate is also encompassed.
The distinction between a grease and a plastic solid is a subjective one. A general rule, and the one adopted herein, is that a grease is easily deformed by hand at 25.degree. C. A plastic requires the force of a machine to cause flow at 25.degree. C. The numeric values adopted reflect this distinction.
A grease is defined as a Bingham plastic that has a yield stress between 10 and 15,000 Pa at 25.degree. C. A silicone grease is a material that contains at least 20 percent diorganosiloxane and exhibits the flow properties of a grease.
A plastic solid is defined as a Bingham plastic that has a yield stress greater than 15,000 Pa at 25.degree. C. A silicone plastic is a material that contains at least 20 percent diorganosiloxane and exhibits the flow properties of a plastic solid.
Silicone rubber is another category of interest. A rubber is a solid material that deforms elastically (elastomeric solid) under low shear stress. After the stress is relieved the original shape is resumed. To be considered a rubber herein, a material must deform elastically when subjected to a strain of 1.1 inches/inch, or more, in tension. When a shear stress that is sufficient to cause permanent deformation is applied to a rubber at 25.degree. C., it will fracture, rather than flow like a Bingham plastic. A silicone rubber is a rubber that contains at least 20 percent by weight of diorganosiloxane.
Curable silicone composition, as used herein, refers to a composition that contains at least 20 percent by weight of diorganosiloxane that can cure to form a rubber. The following steps shall be followed to determine if a particular silicone composition is curable: 1. place 50 grams of the composition in a glass 250 ml beaker, 2. heat the beaker and contents in a free convection oven at 200.degree. C. for four hours, 3. remove the beaker and contents and allow to cool, uncovered, for 24 hours, 4. examine the contents of the beaker, if it fits the definition of a rubber contained herein it is curable.
The final category of silicone compounds to be defined is silicone antifoam agents, often called defoamers. A silicone antifoam agent is a material containing diorganosiloxane that is utilized in foam control. No distinction is made herein between the functions of breaking an existing foam and preventing the formation of foam.
One of the first silicone antifoam agents was initially formulated merely to serve as a grease. A purchaser of the grease found it to be an effective antifoam agent. Any silicone compound that has utility in suppressing foam is considered a silicone antifoam agent.
Prior to the present invention, it was thought that once a silicone compound was cured to the point of being a rubber then it would have lost the ability to serve as an antifoam agent. As will be shown, with careful control of processing conditions, silicone rubbers can be used to make silicone antifoam agents.